The magisterium of the Catholic Church is the church's authority or office to give authentic interpretation of the word of God, "whether in its written form or in the form of Tradition". According to the 1992 Catechism of the Catholic Church, the task of interpretation is vested uniquely in the Pope and the bishops, though the concept has a complex history of development. Scripture and Tradition "make up a single sacred deposit of the Word of God, which is entrusted to the Church", and the magisterium is not independent of this, since "all that it proposes for belief as being divinely revealed is derived from this single deposit of faith".
Solemn and ordinary
The exercise of the Catholic Church's magisterium is sometimes, but only rarely, expressed in the solemn form of an ex cathedra papal declaration, "when, in the exercise of his office as shepherd and teacher of all Christians, in virtue of his supreme apostolic authority, [the Bishop of Rome] defines a doctrine concerning faith or morals to be held by the whole Church," or of a similar declaration by an ecumenical council. Such solemn declarations of the church's teaching involve the infallibility of the Church.
Pope Pius IX's definition of the Immaculate Conception of Mary and Pope Pius XII's definition of the Assumption of Mary are examples of such solemn papal pronouncements. Most dogmas have been promulgated at ecumenical councils. Examples of solemn declarations by ecumenical councils are the Council of Trent's decree on justification and the First Vatican Council's definition of papal infallibility.
The Catholic Church's magisterium is exercised without this solemnity in statements by popes and bishops, whether collectively (as by an episcopal conference) or singly, in written documents such as catechisms, encyclicals, and pastoral letters, or orally, as in homilies. These statements are part of the ordinary magisterium of the church.
The First Vatican Council declared that, "all those things are to be believed with divine and Catholic faith which are contained in the Word of God, written or handed down, and which the Church, either by a solemn judgment or by her ordinary and universal teaching magisterium, proposes for belief as having been divinely revealed."
The Second Vatican Council declared further that not everything contained in the statements of the ordinary magisterium is infallible, but the Catholic Church holds that the Church's infallibility is invested in the statements of its universal ordinary magisterium: "Although the bishops, taken individually, do not enjoy the privilege of infallibility, they do, however, proclaim infallibly the doctrine of Christ on the following conditions: namely, when, even though dispersed throughout the world but preserving for all that amongst themselves and with Peter's successor the bond of communion, in their authoritative teaching concerning matters of faith or morals, they are in agreement that a particular teaching is to be held definitively and absolutely."
Such teachings of the ordinary and universal magisterium are obviously not given in a single specific document. They are teachings upheld as authoritative, generally for a long time, by the entire body of bishops. Examples given are the teaching on the reservation of ordination to males and on the immorality of procured abortion.
Even public statements by popes or bishops on questions of faith or morals that do not qualify as "ordinary and universal magisterium" have an authority that Catholics are not free to merely dismiss. They are required to give that teaching religious submission:
Etymology
The word "magisterium" is derived from Latin magister, which means "teacher" in ecclesiastical Latin. (It originally had a more general meaning, and could designate president, chief, director, superintendent, etc., and was only rarely a tutor or instructor of youth.) The noun magisterium refers to the office of a magister. Thus the relationship between magister and magisterium is the same as the relationship in English between "president" and "presidency".
Since the time of Pope Pius XII, the word "magisterium" has also been used to refer to the persons who hold this office.